Improve printing of symmetric matrices when used in constraints

docs/src/manual/constraints.md

@@ -1462,17 +1462,17 @@ julia> Z = [X[1, 1] X[1, 2]; X[1, 2] X[2, 2]]
 
 julia> @constraint(model, LinearAlgebra.Symmetric(Z) >= 0, PSDCone())
 [X[1,1]  X[1,2]
- X[1,2]  X[2,2]] ∈ PSDCone()
+ ⋅       X[2,2]] ∈ PSDCone()
 ```
 
 Note that the lower triangular entries are ignored even if they are
 different so use it with caution:
 ```jldoctest con_psd
 julia> @constraint(model, LinearAlgebra.Symmetric(X) >= 0, PSDCone())
 [X[1,1]  X[1,2]
- X[1,2]  X[2,2]] ∈ PSDCone()
+ ⋅       X[2,2]] ∈ PSDCone()
 ```
-(Note the `(2, 1)` element of the constraint is `X[1,2]`, not `X[2,1]`.)
+(Note that no error is thrown, even though `X` is not symmetric.)
 
 ## Complementarity constraints
 

src/print.jl

@@ -909,6 +909,8 @@ function function_string(
             end
             if A isa LinearAlgebra.Symmetric && i > j
                 line *= "\\cdot"
+            elseif A isa LinearAlgebra.UpperTriangular && i > j
+                line *= "\\cdot"
             else
                 line *= function_string(mode, A[i, j])
             end
@@ -923,6 +925,19 @@ function function_string(mode, constraint::AbstractConstraint)
     return function_string(mode, f)
 end
 
+# A special case for symmetric matrix constraints. Since the shape is
+# SymmetricMatrixShape, we know that MOI has been passed the upper triangle of
+# the matrix. We can make this clearer to users by printing the
+# LinearAlgebra.UpperTriangular. There shouldn't be any cases in which the
+# constraint function becomes ambiguous.
+function function_string(
+    mode,
+    constraint::VectorConstraint{F,S,SymmetricMatrixShape},
+) where {F,S}
+    f = reshape_vector(jump_function(constraint), shape(constraint))
+    return function_string(mode, LinearAlgebra.UpperTriangular(f))
+end
+
 function function_string(mode::MIME, p::NonlinearExpression)
     nlp = nonlinear_model(p.model)::MOI.Nonlinear.Model
     expr = nlp[index(p)]

src/sd.jl

@@ -125,8 +125,8 @@ julia> a = [x 2x; 2x x];
 julia> b = [1 2; 2 4];
 
 julia> cref = @constraint(model, Symmetric(a - b) in PSDCone())
-[x - 1    2 x - 2
- 2 x - 2  x - 4] ∈ PSDCone()
+[x - 1  2 x - 2
+ ⋅      x - 4] ∈ PSDCone()
 
 julia> jump_function(constraint_object(cref))
 3-element Vector{AffExpr}:

test/test_print.jl

@@ -605,7 +605,7 @@ Subject to
  con : a + b - 10 c + c1 - 2 x $le 1
  a*b $le 2
  [a  b
-  b  x] $inset $(PSDCone())
+  ⋅  x] $inset $(PSDCone())
  [a, b, c] $inset $(MOI.PositiveSemidefiniteConeTriangle(2))
  [a  b
   c  x] $inset $(PSDCone())
@@ -1094,4 +1094,20 @@ function test_invalid_references()
     return
 end
 
+function test_symmetric_constraint()
+    model = Model()
+    @variable(model, x[1:2, 1:2], Symmetric)
+    @test function_string(MIME("text/plain"), x) ==
+          "[x[1,1]  x[1,2]\n x[1,2]  x[2,2]]"
+    @test function_string(MIME("text/latex"), x) ==
+          "\\begin{bmatrix}\nx_{1,1} & x_{1,2}\\\\\n\\cdot & x_{2,2}\\\\\n\\end{bmatrix}"
+    c = @constraint(model, x in PSDCone())
+    o = constraint_object(c)
+    @test function_string(MIME("text/plain"), o) ==
+          "[x[1,1]  x[1,2]\n ⋅       x[2,2]]"
+    @test function_string(MIME("text/latex"), o) ==
+          "\\begin{bmatrix}\nx_{1,1} & x_{1,2}\\\\\n\\cdot & x_{2,2}\\\\\n\\end{bmatrix}"
+    return
+end
+
 end  # TestPrint